The disclosures herein relate generally to knee implants and more particularly to a trial stem including an offset which can be used to locate the trial baseplate position medially or laterally.
Primary tibial implants often fail and must be replaced with a revision implant. An often encountered problem confronting a revision implant is bone loss and implant stability. The result of bone loss is that it precludes attaching a replacement implant directly to the proximal end of the tibia in the same manner as a primary implant by a fastening device or cement. Instead, the proximal end of the tibia is resected and a bore is formed using the intramedullary canal as a guide through the resected proximal end of the tibia to permit the placement of an implant stem. A baseplate is attached to the stem abutting the resected end of the tibia to receive and support an articulating surface.
The anatomy of the human tibia is variable. The tibia comprises an outer layer of hard cortical bone and an inner filling of relatively soft cancellous bone. The strength of the tibia is primarily derived from the cortical bone and should be preserved to support implants. Normally, the intermedullary canal is not in the actual center of the proximal tibia. As a result, when the bore is formed using the intramedullary canal, the stem which is placed in the bore may not be centered within the proximal tibia. Because the baseplate is mounted on the stem, the baseplate may not be ideally positioned with respect to the resected tibial surface. This can result in an overhang of the baseplate relative to the resected end of the tibia and a resulting irritation of soft surrounding tissue. The baseplate must therefore be kept within the confines of the tibia. As a result, it is often necessary to provide an offset, either medially or laterally, in order to properly orient the baseplate on the proximal tibia relative to the stem.
A surgeon may prefer to press-fit a tibial stem in the canal instead of cementing. In that case, the entire tibial canal is subsequently reamed out, i.e. all cancellous bone is removed. A good cortical fit is desired. Reaming is accomplished by using various sized reamers and progressively increasing the reamer size to reach the desired canal diameter and depth. Subsequently, the surgeon trials with various size straight tibial stems having a baseplate threadably attached, to find a stable press-fit with the cortical bone, but may find that the baseplate overhangs the proximal tibia. To correct for the overhang, the surgeon downsizes from the press-fit stem to a cemented stem in an effort to properly locate the baseplate, and then cements the implants in position.
If the bore is offset to compensate for the off-center canal within the tibia, the implant may be supported on some surface areas by the hard cortical bone and on other surface areas by the relatively softer cancellous bone. This is undesirable and may require a filler cement to stabilize and secure the stem. Also, if a subsequent replacement is required, the filler must be removed.
One attempt to overcome the above-mentioned problems with tibial stem implants is addressed in U.S. Pat. No. 5,133,760, which provides a universal modular prosthetic stem extension which may be installed on a prosthesis in a multiplicity of different orientations to compensate for a multiplicity of patient conditions. The stem includes a coupling mechanism allowing the stem to be rotated to any one of a multiplicity of rotative positions with respect to a prosthetic base so that the stem may be fixed in position relative to the base.
Another device is disclosed in U.S. Pat. No. 5,271,737 which comprises a combination baseplate fixed to an offset, straight tibial stem. The base includes an inferior surface for abutting a resected surface of the patient's tibia. The longitudinal center axis of the straight tibial stem extends from the inferior surface of the base and is offset from a center of the base. Interestingly, the offset places the stem in position to extend into the canal of the tibia so that it does not interfere with the cortical bone. As a result of the fixed arrangement, one baseplate and stem is required for a medial offset and another is required for a lateral offset.
A further device is disclosed in U.S. Pat. No. 5,290,313, which comprises a modular prosthesis system including a modular stem which has an attachment section for attachment to the base, a main body section for implanting into the canal in the tibia, and an angled transition section. The attachment section and main body section each include a respective longitudinal axis. These axes are parallel to each other and spaced apart to provide an offset therebetween. The offset is substantial, such that the axis of the attachment section intersects the transition section.
However, although offset stem implants are provided, accuracy requires a satisfactory trial stem and trial baseplate which simulate the offset of the stem and baseplate implants.
Therefore, what is needed is an offset trial stem which can be attached to a trial baseplate so that when the offset trial stem is attached to the trial baseplate and set into the canal, they are in the same orientation as the offset tibial stem and baseplate will be when implanted.